1. Field of the Invention
The present invention relates to a vibration motor, and more particularly, to a flat-type vibration motor.
2. Description of the Related Art
Flat-type vibration motors are used to attain vibratory motion for silent call of personal information appliances, such as mobile communication terminal.
A demand for the flat-type vibration motors are being increased as they are being used in mobile communication terminals for vibration means. A miniaturization and a lightweight are demanded for the mobile communication terminal, such that the vibration motor also needs to be compact but have a big vibratory motion.
Various types of flat-type vibration motors are provided to meet these demands.
A flat-type vibration motors according to the related art will now be described with reference to FIG. 1 through FIG. 4.
FIG. 1 is a cross-sectional view schematically showing a structure of a related art vibration motor. FIG. 2A is a plan view showing a structure of the related art vibration motor. FIG. 2B is a bottom view showing a structure of the rotor of the related art vibration motor. FIG. 2C is an equivalent circuit diagram showing connections between winding coils and a commutator of the related art vibration motor.
Referring to FIGS. 1, 2A and 2B, the related art vibration motor includes: a lower case 120 provided with a lower board 121; an upper case 110 to be coupled with the lower case 120; a four-pole magnet 140 disposed on the lower case 120; a shaft 130 supported between the lower case 120 and the upper case 110; a rotor 150 eccentrically supported by the shaft 130 to rotate; two coils 151 disposed in the rotor 150 at an included angle of 120° therebetween based on a center thereof; a commutator 152 formed with eight segments and located beneath the rotor 150, the upper board 153 being formed on a bottom of the rotor 150; a pair of brushes 160 having bottoms attached on the lower board 121 and tops contacted with the segments of commutator 150, wherein the tops of the brushes 160 are arranged to have an included angle of 90° therebetween; and a weight 154 interposed between the coils 151.
A reference numeral 131 denotes a bearing for supporting a rotation of the shaft 130 and a reference numeral 132 denotes a washer to be coupled to the shaft 130.
In the commutator 152, a first and a fifth segments, a second and a sixth segments, a third and a seventh segments, and a fourth and an eighth segments are respectively connected to make electric current passages.
One of the two coils 151 is connected between the (second, sixth) segments and the (fourth, eighth) segments of the commutator 152.
The other of the two coils 151 is connected between the (first, fifth) segments and the (third, seventh) segments of the commutator 152.
Equivalent circuit diagrams of connections between the coils 151 and each segment of the commutator 152 are showed in FIG. 2C.
The related art vibration motor constructed as above operates such a manner that the rotor 150 is driven by attractive/repulsive forces between the magnet 140 and the coils 151.
FIGS. 3A through 3D are wiring diagrams to explain a operation of the related art vibration motor of which brushes have an included angle of 90°.
Referring to FIG. 3A, an anode of the brushes 160 is in contact with the seventh segment of the commutator 152 and a cathode of the brushes is in contact with the first segment of the commutator 152, such that a current flows through the coil 151 that is connected between the first and the seventh segments. A magnetic flux generated by the current flow interacts with a magnetic flux of the magnet 140 such that generates the attractive/repulsive force to rotate the rotor 150.
A contacting position between the brushes 160 and the commutator 152 is varied as the rotor 150 is rotating. FIG. 3B through 3D show circuits according to the variation of the contacting position. Operation characteristics of the circuits are the same as those of FIG. 3A.
The included angle between the brushes 160 can be varied though it has been described with 90° as shown in FIGS. 3A through 3D.
FIGS. 4A and 4B are wiring diagrams illustrating a problem of the related art vibration motor having a brush angle greater than 90°, and FIGS. 5A and 5B are wiring diagrams illustrating a problem of the related art vibration motor having the brush angle less than 90°
Referring to FIGS. 4A and 4B, in case the included angle of the brush pair 160 is greater than 90°, the anode of the brushes 160 may contact with the fifth segment and the cathode of the brushes 160 may contact with the eighth segment. In this case, the coil 151 is not applied with an electrical potential and thus the magnetic flux is not generated in the coil 151, thereby not allowing the rotor 150 to rotate.
Referring to FIGS. 5A and 5B, in case the included angle of the brush pair 160 is less than 90°, the anode of the brushes 160 may contact with the sixth segment and the cathode of the brushes 160 may contact with the seventh segment. In this case, the coil 151 is not applied with an electrical potential and thus the magnetic flux is not generated in the coil 151, thereby not allowing the rotor 150 to rotate.
In other words, the related art vibration motor having two coils and eight-segment commutator has a drawback in that death point occurs when the brush angle is out of 90°, such that the motor is not driven.
Another type of the related art vibration motor having two coils and a six-segment commutator is disclosed in Japanese Patent Laid-Open Publication No. 13-104882.
The disclosed vibration motor includes a four-pole magnet, in which North and South poles are alternately formed, and brushes arranged at a brush angle of 90°, and a pair of coils are arrange at an angle of 120° based on a center thereof. So, the disclosed vibration motor has a similar structure to that of the vibration motor shown in FIG. 1, except that it has the six-segment commutator.
FIGS. 6A, 6B and 6C are graphs showing an impressed current, a counter electromotive force, and a torque with respect to rotation angle of the rotor.
Referring to FIG. 6C, it is found that the related art vibration motor disclosed in Japanese Patent Laid-Open Publication No. 13-104882 has a large torque ripple of 50%.
Therefore, the disclosed vibration motor requires a high voltage input for operation owing to the large torque ripple, thereby reducing productivity.
As described above, the related art vibration motor having two coils and eight-segment commutator has a drawback in that the death point occurs when the brush angle is out of 90°, such that the motor is not driven. On the other hand, the related art vibration motor having two coils and six-segment commutator has a drawback that has a large torque ripple.